1. Field of the Invention
The present invention relates to a polymer electrolyte membrane for a fuel cell, a method for manufacturing the polymer electrolyte membrane, and a membrane-electrode assembly incorporating the polymer electrolyte membrane for a fuel cell and a fuel cell system incorporating the membrane-electrode assembly. More particularly, the present invention relates to a polymer electrolyte membrane having large moisture retention and fuel cross-over inhibition properties and a method for manufacturing the polymer electrolyte membrane, and a membrane-electrode assembly incorporating the polymer electrolyte membrane for a fuel cell and a fuel cell system incorporating the membrane-electrode assembly.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction between an oxidant and a fuel such as hydrogen, or a hydrocarbon-based material such as methanol, ethanol, and natural gas.
Such a fuel cell is a clean energy source that can replace fossil fuels. The fuel cell is typically constructed with a stack composed of unit cells that produces various ranges of power output. Since it has an approximately four to ten times higher energy density than a small lithium battery, it has been highlighted as a small portable power source.
Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel.
The polymer electrolyte fuel cell has advantages of high energy density and high power, but it also has problems in the need for exceptionally careful handling of hydrogen gas and in its requirement for accessory facilities such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like in order to produce hydrogen as the fuel gas.
On the contrary, a direct oxidation fuel cell has a lower energy density than that of the gas-type fuel cell but has the advantages of easy handling of the liquid-type fuel, a low operation temperature, and no need for additional fuel reforming processors. Therefore, it has been acknowledged as an appropriate system for a portable power source for small and common electrical equipment.
In the above-mentioned fuel cell system, the stack that generates electricity substantially includes several to scores of unit cells stacked adjacent to one another, and each unit cell is constructed with a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly is constructed with an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane.
A fuel is supplied to the anode and is adsorbed on catalysts of the anode, and the fuel is oxidized to produce protons and electrons. The electrons are transferred into the cathode via an external circuit, and the protons are transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode, and then the oxidant, protons, and electrons react on the catalyst of the cathode to produce electricity along with water.